Laser radiation state selection

As an example, we mention the detection of iodine atoms in their P3/2 ground state with a 3 + 2 multiphoton ionization process at a laser wavelength of 474.3 run. Excited iodine atoms ( Pi/2) can also be detected selectively as the resonance condition is reached at a different laser wavelength of 477.7 run. As an example, figure B2.5.17 hows REMPI iodine atom detection after IR laser photolysis of CF I. This pump-probe experiment involves two, delayed, laser pulses, with a 200 ns IR photolysis pulse and a 10 ns probe pulse, which detects iodine atoms at different times during and after the photolysis pulse. This experiment illustrates a frindamental problem of product detection by multiphoton ionization with its high intensity, the short-wavelength probe laser radiation alone can photolyse the... 

Let us discuss briefly the peculiarities of the excited states. Highly excited (Rydberg) states form a separate class of excited states. Experimentally, using, for example, selective laser radiation, it is possible to excite one electron to the states described by very high n values. Rydberg states of atoms are observed in interstellar space. Rydberg atoms possess a number of pecularities their sizes may be very large, comparable to the size of... 

Most of the basic ideas of atomic or molecular state selection by optical means were formulated near the advent of the laser (e.g. Kastler, 1950, and Dehmelt and Jefferts, 1962). However, widespread application of the technique only took place in the late 1970s and early 1980s with the availability of dependable tuneable CW and pulsed lasers. Only laser radiation carries a sufficiently high spectral density for significant manipulation of the thermal population of atomic (or molecular) levels in a beam. 

Figure B2.3.8. Energy-level schemes describing various optical methods for state-selectively detecting chemical reaction products left-hand side, laser-induced fluorescence (LIF) centre, resonance-enhanced multiphoton ionization (REMPI) and right-hand side, coherent anti-Stokes Raman spectroscopy (CARS). The ionization continuum is denoted by a shaded area. The dashed lines indicate virtual electronic states. Straight arrows indicate coherent radiation, while a wavy arrow denotes spontaneous emission.

Chemical lasers are complex nonequilibrium molecular systems governed by an intricate interplay between a variety of chemical, radiative, and collisional relaxation processes. Many of their kinetic properties are reflected by the temporal, spectral, and power characteristics of the out-coupled laser radiation. For example, threshold time measurements and other gain experiments have provided detailed information on vibrational distributions of nascent reaction products. Another, more qualitative, example Single-line and simultaneous multiline operation indicate, respectively, whether the lasing molecules are rotationally equilibrated or not. Besides their practical applications, chemical lasers are widely used as means of selective excitation in state-to-state kinetic studies. On the other hand, many experimental and theoretical studies have been motivated by the wish to understand and improve the mechanism of chemical laser operation. 

The preparation of nonequilibrium level or species populations is the first step in any kinetic experiment. The introduction of lasers to chemical research has opened up new possibilities for preparing, often state-selectively, the initial nonequilibrium states. However, the subsequent time evolution of the molecular populations occurs almost invariably along several relaxation pathways. Some of which, like intra- and intermolecular vibrational energy transfer in infrared multiphoton absorption experiments, may interfere with the exciting laser pulse and/or with the specific process investigated. In such cases, as in chemical laser research, one has to interpret the behavior of complex nonequilibrium molecular systems in which the laser radiation plays of course a major role. This establishes the link between the present article and the general subject of this volume. 

In 1970 the first report of the molecular hydrogen laser opened up a decade of activity in VUV laser development, which included the appearance of rare gas excimer and exciplex lasers and the achievement of tunable coherent radiation in the Lyman-a region via harmonic generation. The surge of activity in the development of VUV lasers arose in part from the uniqueness of the VUV region, in part from the ultimate interest in X-ray lasers and, from our perspective, from the exciting prospects in spectroscopy and molecular dynamics promised by narrow linewidth, tunable, high-power VUV laser pulses for state-selective studies. Here we review the principles on which VUV lasers are based. 

The experimental scheme used in these experiments is quite simple in its principle (see Figure 1). The Rydberg atoms are prepared by laser excitation of an atomic beam. The laser radiation is attenuated until one makes sure that no more than one atom at a time is prepared in the cavity. The optical selection rules result in a preparation of low angular momentum Rydberg levels (practically s, p or d states with Jl = 0, 1 or 2 depending upon the number of photons involved in the optical transition). 

Laser radiation is monochromatic and in many cases it also is tuneable these two characteristics together provide the basis for high-resolution laser spectroscopy. The interaction between laser radiation and molecules can be very selective (individual quantum states can be selected), permitting chemists to investigate whether energy in a particular type of molecular motion or excitation can influence its reactivity. Photochemical processes can be carried out with sufficient control that one can separate isotopes, or even write fine fines (of molecular dimensions) on surfaces. 

When an atom or molecule interacts with a photon of sufficient energy, ionization may occur through removal of an electron. Because the ionization potential of most small molecules is larger than 8 eV, highly energetic photons in the VUV are required to induce ionization through one-photon absorption. Tuneable radiation may be provided by synchrotron sources and by (normally very inefficient) non-linear conversion of radiation from visible or UV lasers. However, direct one-photon ionization exhibits little state selectivity, i.e. ionization of normally several vibrational and rotational levels of the electronic ground state occurs. Consequently, in the photo-ion one also encounters a superposition of several vibrational and rotational levels (determined mostly by the Franck-Condon factors between the initial and final vibrational states). 

Over-the past decade, not only have pulse durations decreased from 10 to 10" s but there has been a dramatic increase in the tunability of lasers, such that tunable coherent radiation can now span the VUV to the very long wavelength laser radar. Femtosecond spectroscopy, like most advances, has begun in the visible region and considerable research and development is necessary to expand this present spectral range around 600 nm (4). However, it is also the case that for many problems in photo dynamics, for which the state selectivity or the nature of the optically prepared initial state is of paramount importance, the spectral line-width (Av) of the pulse must remain narrow. Thus the transform-limited bandwidth relationships (AvA K) govern the temporal properties of the laser pulse and, for example, a 5 ns pulse of 0.01 cm" linewidth prepares a different ensemble than a 300 fs pulse of 26 cm linewidth at the same wavelength. 

The principal limitation of optical pumping state selection, particularly to light atoms, is the relatively limited wavelength range of cw lasers. However, frequency doubled cw laser radiation is now available [53]. It is interesting to speculate on... 

Probably the first suggestion for utilizing the properties of laser light (the high intensity and short duration of radiation pulses) was (Letokhov 1969) to use the vibrationally mediated photodissociation of molecules via an excited repulsive electronic state with noncoherent isotope-selective saturation of the vibrational transition (Fig. 11.2). The isotope-selective two-step photodissociation of molecules consists of pulsed isotope-selective excitation of a vibrational state in the molecules by IR laser radiation and subsequent pulsed photodissociation of the vibrationally excited molecules via an excited electronic state by a UV pulse (Fig. 11.2(a)) before the isotope selectivity of the excitation is lost in collisions. Selective two-step photodissociation of molecules is possible if their excitation is accompanied by a shift of their continuous-wave electronic photoabsorption band. In that case, the molecules of the desired isotopic composition, selectively excited by a laser pulse of frequency uji, can be photodissociated by a second laser pulse of frequency uj2 selected to fall within the region of the shift where the ratio between the absorption coefficients of the excited and unexcited molecules is a maximum (Fig. 11.2(b)). 

Ambartzumian, R. V., Letokhov, V. S., Ryabov, E. A., and Chekalin, N. V. (1974). Isotope-selective chemical reaction of BCI3 molecules in a strong IR laser field. Journal of Experimental and Theoretical Physics Letters, 20, 273-274. Ambartzumian, R. V., Bekov, G. L, Letokhov, V. S., and Mishin, V. I. (1975a). Excitation of high-lying states of Na atoms by means of tunable laser radiation and autoionization of these states in an electric field. Journal of Experimental and Theoretical Physics Letters, 21, 279-281. 

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